’s Epoch value (used when converting positions to or from the
"system). It should be set to the difference, in seconds, between the UT1 and UTC timescales at the moment in time represented by the SkyFrame
’s Epoch attribute. The value to use is unpredictable and depends on changes in the earth
’s rotation speed. Values for UT1-UTC can be obtained from the International Earth Rotation and Reference Systems Service (IERS) at http://www.iers.org/.
Currently, the correction is always less than 1 second. This is ensured by the occasional introduction of leap seconds into the UTC timescale. Therefore no great error will usually result if no value is assigned to this attribute (in which case a default value of zero is used). However, it is possible that a decision may be taken at some time in the future to abandon the introduction of leap seconds, in which case the DUT correction could grow to significant sizes.